The underline position for Japanese/Korean vertical text is wrong. Horizontal text uses an alphabetic baseline as the basis for the underline's position, but this is incorrect for Japanese vertical text. In that case the underline needs to be to the left of the glyphs themselves (past the descent).
This could get tricky to implement given that vertical metrics on OS X are including the leading in the descent value. We may have to operate using glyph bounds, since we know the glyphs are square.